The blown-out windows in the gothic-style mansion overlook a sandbagged courtyard strewed with 4x4s, fallen trees, stray dogs and empty bullet casings. The crack of machine gunfire competes with the steady boom of mortars as men hurry in and out the building ferrying food and ammunition to their comrades inside.

A few days ago this extravagant fortress perched atop a hill in the east of the Yemeni capital was the tranquil abode of Sadeq al-Ahmar, leader of the Hashid, the country's wealthiest and most powerful tribe. Now it is a bullet-pocked, crumbling garrison shrouded in black smoke from mortar fire, and home to hundreds of Kalashnikov-wielding tribesmen who are battling it out in the streets with President Ali Abdullah Saleh's security forces in an effort to end his three decade-long rule.

The sound of gunfire and exploding mortar shells has echoed through Sana'a for four days now since Saleh refused on Sunday to accept a Gulf Co-operation Council-negotiated resolution to Yemen's four-month crisis that would have eased him out of power with immunity. More than 100 people have been killed.

The clashes between Saleh's republican guard and members of the Hashid tribe, are the bloodiest Yemen has seen since protests began in January and briskly fanning the fears of civil war.

After a brief lull on Wednesday night, fighting seemed to intensify on Thursday as the two sides, now separated only by a few residential blocks, fired anti-aircraft missiles at each other as they scrambled for control of government buildings and the airport, their battle slowly encroaching further towards the centre of the city.

A huge explosion rang out just after midday, rattling the windows of houses across the capital after a stray shell hit a munitions depot. A defence ministry official said at least 28 people, most of them civilians, had died. A few hours later Suhail TV, the country's only opposition television station was taken off air after its headquarters was shelled by government forces. Overall more than 40 people were reported killed in San'a yesterday.

With the violence flaring, panic has begun to grip Sana'a. Long lines of cars and buses with bags strapped to the roofs were seen filtering out of the city. Those staying put have started hoarding, withdrawing cash, and filling buckets with petrol and barricading themselves indoors.

The foreign secretary, William Hague, urged Saleh to hand over power, reduced embassy staff and warned all British expatriates to leave Yemen immediately. "British nationals should not remain [in Yemen] ... I cannot stress this too strongly," he said.

The United States also ordered non-essential personnel and family members of staff to leave the country. "The security threat level in Yemen is extremely high due to terrorist activities and civil unrest," the state department said.

Shopkeeper Fares al-Mana, who was leaving the Yemeni capital, said the confrontations were spreading. "It's no longer possible to stay in Sana'a," he said.

In an act that suggests the president's patience is running thin, Saleh ordered the arrest of Ahmar, whose men now control of several ministry buildings near his compound including the trade and tourism ministries, as well as the offices of the state news agency Saba.

The tribal chief remained defiant. In an interview with al-Jazeera on Thursday he called Saleh a liar and said he had captured 70 government troops. "I'm protected by Hashid and other tribesmen and even by army soldiers, I have 70 soldiers captive. Ali Abdullah Saleh is a liar, liar, liar. We are firm. He will leave this country barefoot," he said. Back at Ahmar's fortress, his guards were bracing themselves for another night of fighting. "This started as self-defence but now we're fighting for his downfall," said Sheikh Mohammed al-Farasi, a scrawny man with bloodshot eyes loading his AK-47 with cartridges.

"There's no bigger shame for a tribal leader than having his house attacked. The only way this can end is if Saleh goes, the tribes have said enough is enough."

Attempts at mediation have thus far failed. On Tuesday a sheikh sent by Saleh to try to defuse the situation was killed when Ahmar's house came under heavy fire from government forces.

"What we're witnessing now is a battle between the two most powerful families in Yemen, a conflict that has been brewing for several years which because of Saleh's stubbornness has come to its head," said Abdullah al-Faqah, professor of politics at Sana'a University. "This was a foolish fight for him [Saleh] to pick."

The Ahmar clan head Hashid, the largest tribal confederation in Yemen. Saleh had managed to keep the family patriarch, Abdullah, onside during his rule, but since he died in December 2007, power has passed to the 10 Ahmar brothers.

Four brothers from the most significant threat to Saleh's rule. They include Sadiq, the head of the Hashid tribal; Hamir, the deputy speaker of parliament; Hussein, a powerful tribal leader; and, most significant of all, Hamid, a business tycoon and founder of the opposition party Islah.

Hamid has positioned himself as a potential successor to Saleh and accuses the president of violating the constitution by turning Yemen into his family enterprise.

Hamid is now thought to be bankrolling the opposition as well as supporting the upkeep of the hundreds of thousands of anti-government protesters camping out in Sana'a's Change Square.

General Ali al-Mohsen, one of Yemen's most powerful military leaders who defected in March and so far steered clear of the violence but called on the armed forces to defy the president. "Beware of following this madman who is thirsty for more bloodshed," he said.